High-lift systems are utilized on the wings of aircraft to increase lift or drag during take-offs and landings. One type of high-lift system uses flaps on the trailing edge of the wings. The flaps are moveable surfaces that may be extended during take-offs and landings, and retracted at cruising speeds.
There are a variety of actuation systems used to extend and retract trailing-edge flaps on a wing. One particular type of actuation system uses a drive station on each side of the flap. When a drive station is installed on either side of a flap with no mechanical interaction between the drive stations, this type of actuation system is referred to as a “distributed” actuation system. A drive station includes a motor, a gear train, and a drive screw that connects to the flap through an actuator arm. The motor (e.g., an electric motor) in a drive station turns the drive screw in a forward or reverse direction through the gear train. The drive screw converts the rotation motion of the motor and gear train into a linear motion to impart movement on the actuator arm. As the actuator arm is pushed or pulled by the drive screw, the flap connected to the actuator arm is extended or retracted. It is also possible to have an actuator arrangement in a rotary configuration with a rotary output rather than a linear output.
A mechanical malfunction may occur when the drive station on one side of the flap becomes disconnected and is no longer able to impart movement on the flap. After this failure occurs, the drive station on one side of the flap would bear the full load of holding the flap in the selected position. For example, when the aircraft is taking off or landing, the flaps are held at selected positions (e.g., fully extended or partially extended) to increase drag or lift. When the flap is being held in position by one side, the flap can actually twist when under sufficient airloads. This condition is referred to as a “freewheeling skew” failure.
If the flap is actively being extended or retracted when the malfunction occurs (e.g. a jam or a disconnect within the drive station), then the flap will be driven from only one side which can cause the flap to twist. This condition is referred to as a “powered skew” failure. Both of these failure conditions can damage the high-lift system of the aircraft.